1. Field of the Invention
Exemplary aspects of the present invention relate to an image forming apparatus and an image forming method, and more particularly, to an image forming apparatus and an image forming method using a plurality of image forming devices for forming respective toner images.
2. Description of the Related Art
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile functions, typically form an image on a recording medium (e.g., a transfer sheet) based on image data using electrophotography. Thus, for example, a charger uniformly charges a surface of an image carrier; an optical writer emits a light beam onto the charged surface of the image carrier to form an electrostatic latent image on the image carrier according to the image data; a development device supplies toner particles to the electrostatic latent image formed on the image carrier to make the electrostatic latent image visible as a toner image; the toner image is directly transferred from the image carrier onto a transfer sheet in a direct transfer method or is indirectly transferred from the image carrier onto a transfer sheet via an intermediate transfer member in an indirect transfer method; a cleaner then cleans the surface of the image carrier after the toner image is transferred from the image carrier onto the transfer sheet; and finally, a fixing device applies heat and pressure to the transfer sheet bearing the toner image to fix the toner image on the transfer sheet, thus forming the image on the transfer sheet.
Such image forming apparatus may include a plurality of image forming devices, each of which includes the charger, the image carrier, the development device, and the cleaner, so as to form a color toner image on a transfer sheet. For example, the plurality of image forming devices forms toner images in respective colors and the toner images are sequentially transferred onto a transfer sheet being conveyed in such a manner that the toner images are superimposed on the transfer sheet to form a color toner image on the transfer sheet in the direct transfer method. Alternatively, the toner images formed by the plurality of image forming devices, respectively, are transferred onto a rotating intermediate transfer member sequentially in such a manner that the toner images are superimposed on the intermediate transfer member, and then the superimposed toner images are collectively transferred from the intermediate transfer member onto a transfer sheet being conveyed to form a color toner image on the transfer sheet in the indirect transfer method.
Such image forming apparatus can form a toner image properly when the image forming device is new. However, over time, a charge amount of a developer used in the image forming device decreases, resulting in formation of a low-quality solid image and a low-quality halftone image having a low toner density. Especially, the low-quality image having the low toner density may appear as a rough image.
To address this problem, a technology to set a proper transfer electric current for transferring a toner image onto a transfer sheet that varies according to a number of sheets printed is proposed. Such technology is applicable to an image forming apparatus including a single image forming device, but is not applicable to an image forming apparatus including a plurality of image forming devices. It is especially difficult to apply such technology to an image forming apparatus using the indirect transfer method, because each of the plurality of image forming devices degrades at different rates and to different degrees. Accordingly, the conditions under which the superimposed toner images are properly transferred from the intermediate transfer member onto a transfer sheet may be different for each of the toner images formed by the plurality of image forming devices and superimposed on an intermediate transfer member.
Further, toner images formed by image forming devices provided upstream in a direction of rotation of the intermediate transfer member are transferred onto the intermediate transfer member and then conveyed past other image forming devices provided downstream from the upstream image forming devices, during which time the toner images are susceptible to various physical actions performed by the other image forming devices. Accordingly, such toner images need to be transferred from the intermediate transfer member onto a transfer sheet under conditions different from the conditions for an image forming apparatus including only a single image forming device.